A crankshaft is a principal component of a reciprocating engine, which produces power by converting reciprocating motion of pistons to rotary motion. Generally, there are two types of crankshafts: those that are manufactured by forging and those that are manufactured by casting. For engines of automobiles such as passenger cars, freight cars, and specialized work vehicles, particularly multiple cylinder engines having two or more cylinders, it is necessary that their crankshafts have high strength and stiffness, and therefore forged crankshafts, which are more capable of meeting the need, are widely used. For multiple cylinder engines of motorcycles, agricultural machines, marine vessels, and the like, forged crankshafts are also used.
In general, forged crankshafts for multiple cylinder engines are manufactured by using, as a starting material, a billet having a circular or square cross section and having a constant cross-sectional area along the entire length, and subjecting the billet to the steps of preforming, die forging, trimming and coining in order. The preforming step includes roll forming and bending, and the die forging step includes block forging and finish forging.
FIG. 1 is a schematic diagram illustrating a typical conventional process for manufacturing a forged crankshaft. A crankshaft 1 illustrated in FIG. 1 is intended to be mounted in a 4-cylinder engine. It is a 4-cylinder 8-counterweight crankshaft that includes: five journals J1 to J5; four crank pins P1 to P4; a front part Fr, a flange F1, and eight crank arms (hereinafter referred to as “crank arms”) A1 to A8 that connect the journals J1 to J5 and the crank pins P1 to P4 to each other, wherein each of the eight crank arms A1 to A8 has a balance weight. Hereinafter, when the journals J1 to J5, the crank pins P1 to P4, and the crank arms Al to A8 are each collectively referred to, a reference character “J” is used for the journals, a reference character “P” for the crank pins, and a reference character “A” for the crank arms.
According to the manufacturing method shown in FIG. 1, the forged crankshaft 1 is manufactured in the following manner. Firstly, a billet 2 shown in FIG. 1(a), which has been previously cut to a predetermined length, is heated by an induction heater or a gas atmosphere furnace and then is subjected to roll forming In the roll forming step, the billet 2 is rolled and reduced in cross section by grooved rolls, for example, to distribute its volume in the longitudinal direction, whereby a rolled blank 103, which is an intermediate material, is formed (see FIG. 1(b)). In the bending step, the rolled blank 103 obtained by roll forming is partially pressed in a press in a direction perpendicular to the longitudinal direction to distribute its volume, whereby a bent blank 104, which is a secondary intermediate material, is formed (see FIG. 1(c)).
Then, in the block forging step, the bent blank 104 obtained by bending is press forged with a pair of upper and lower dies, whereby a forged blank 105 having a general shape of a crankshaft (forged final product) is formed (see FIG. 1(d)). Then, in the finish forging step, the block forged blank 105 obtained by block forging is further processed by press forging the block forged blank 105 with a pair of upper and lower dies, whereby a forged blank 106 having a shape in agreement with the shape of the crankshaft is formed (see FIG. 1(e)). In the block forging and the finish forging, excess material flows out as a flash from between the parting surfaces of the dies that oppose each other. Thus, the block forged blank 105 and the finish forged blank 106 have large flashes 105a, 106a, respectively, around the formed shape of the crankshaft.
In the trimming step, the finish forged blank 106 with the flash 106a, obtained by finish forging, is held by dies from above and below and the flash 106a is trimmed by a cutting die. In this manner, the forged crankshaft 1 is obtained as shown in FIG. 1(f). In the coining step, principal parts of the forged crankshaft 1, from which the flash has been removed, e.g., shaft parts such as the journals J, the crank pins P, the front part Fr, and the flange F1, and in some cases the crank arms A, are slightly pressed with dies from above and below and formed into a desired size and shape. In this manner, the forged crankshaft 1 is manufactured.
The manufacturing process shown in FIG. 1 is applicable not only to a 4-cylinder 8-counterweight crankshaft as exemplified, but also to a 4-cylinder 4-counterweight crankshaft in which, among 8 crank arms A, the leading first crank arm A1, the trailing eighth crank arm A8, and the two central, fourth and fifth crank arms A4, A5 have balance weights. Also, the same manufacturing process can be applied to crankshafts that are to be mounted in a 3-cylinder engine, an inline 6-cylinder engine, a V-type 6-cylinder engine, an 8-cylinder engine, and the like. It is noted that, when adjustment of the placement angle of the crank pins is necessary, a step of twisting is added after the trimming step.
With such a manufacturing method, it is inevitable that material utilization decreases because large amounts of unnecessary flash, which is not a part of the end product, are generated. Thus, in the manufacturing of a forged crankshaft, it has so far been an important object to inhibit the generation of flash to the extent possible and achieve improvement of material utilization. Examples of conventional techniques that address this object are as follows.
For example, Patent Literature 1 discloses a technique for manufacturing a crankshaft, the technique including: using, as a blank, a stepped round bar having reduced diameter regions at portions to be formed into journals and crank pins of a crankshaft; holding, with dies, a pair of the portions to be formed into journals, between which a portion to be formed into a crank pin is disposed and, in this state, axially moving the opposing dies toward each other to compressively deform the round bar blank; pressing punches against the portion to be formed into a crank pin in a direction perpendicular to the axial direction to place the portion to be formed into a crank pin into an eccentric position; and repeating the above operations in succession for all crank throws, whereby the journals and the crank pins are shaped and the crank arms are roughly shaped.
Patent Literature 2 discloses a technique for manufacturing a crankshaft, the technique including: using, as a blank, a simple round bar; holding one end of the two ends of the round bar with a stationary die and the other end thereof with a movable die, and holding a portion to be formed into a journal with journal dies and portions to be formed into crank pins with crank pin dies; in this state, axially moving the movable die, the journal dies and the crank pin dies toward the stationary die to compressively deform the round bar blank; and moving the crank pin dies in an eccentric direction perpendicular to the axial direction to place the portion to be formed into the crank pin into an eccentric position, whereby the journals and the crank pins are shaped and the crank arms are roughly shaped.
With both the techniques disclosed in Patent Literatures 1 and 2, no flash will be generated, and therefore a significant improvement in material utilization can be expected.